Carrier communication system



June 21, 1938. w. TlDD 2,120,994

I CARRIER COMMUNICATION SYSTEM I Filed Jan. 25, 1936 2 Sheets-Sheet l T g k I/VVE R B V W H. 7700 A 7' TORNE Y June 21, 1938. w. H. TIDD 2, 20,994

CARRIER COMMUNICATION SYSTEM Filed Jan. 25, 1936 2 Sheets-Sheet 2 TELE CIRCUIT AMPLIFIERS RF, REG: RECT /N VE N TOR By um. 7/00 A rroR/L/Ev Patented June 21, 1938 UNlTEDsrATEs QP ATENT OFFICE i CARRIER COMMUNIGATION SYSTEM Warren H. Tidd, White Plains N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, Y., a corporation of New York Application January used as the medium of electrical connection;be--

tween the electrical devices and apparatuses located at different points in the system. These i 5 carrier currents are assigned such frequencies as to bench-interfering with one another, so that each carrier serves as a connecting medium separate and distinct from other carriers superposed upon the line or medium over which the carriers :2 are transmitted. Each of the carriers is modulated inaccordance with the voice currents or other signaling or controlling currents that are to be carried. The carriersthus modulated are transmitted over the toll or long distance line or other common transmission medium such as, for

instance, a pair of coaxial conductors, and are passed through individual arrangements each of which is selective of one carrier frequency to the exclusion of the others,-and each of which-may 1 have associated with it apparatus that serves to reproduce, from amodulated carrier, theyoice, signal or controlling current with which thecarrier was modulated at the distant station. The arrangement which is used to transmit and .re- 'ceive by means of any high frequency wave as distinguished from that which is used for transmitting and receiving by means of a wave of a different frequency is commonly known as a high frequency or carrier channel, or as a frequency selective channel.- v V The present invention is directed particularly to a high frequency system in which intercommunication between the variousstations of the system is effected through the medium of a 54 number of such non-interfering high frequency channels, each of which may be available to all of the stations. Each of the stations of the system is so organized that not only may it establish connection with any other station of the '50 system by way of any of thehigh frequency 55 In order to simplify the system, effect economy 1 tionsmay establish connection. with one another aby'kthe use of alternating'current impulses of different frequencies'whereby operators at each 3,;1936, Serial N0. 60,386 5 Claims, (o gj is-ss) in theapparatus required, and realize certain advantageous features of operation, one of the objects of this invention is to provide an improved means by which each of the distant staof the stations can transmit numberor code designations in'either direction. In the system of my invention all of the communication channels,,in pairs, areavailable to every station. In order to reach a distant office an idle pair of channels is selected and a voice frequency pulse consisting of a com binationof two or more such voice frequencies is superposed upon one of them to: actuate signals at the desired distant ofiice --only, the call being completed thereat manually by an operator in thewell-known manner.

- The scope and purpose of the invention may be the more easily obtained by. considering the fol- 2 lowing description in connection with the attached drawingsin which Fig. 1 shows a trunk at an originating station using high frequency channels derived from a .common medium." which can be used for originating a connection on a straight-forward-basis, Wh A Fig. 2 shows a similar trunk at some terminating ,station connected to the common medium which can be used for completing a connection.

Each of the trunks is shown associated with cord connecting facilities having key transmit- .ting devices connected thereto by which calling or called lines may be extended to each of the trunks and through which .code impulses may be 6 transmitted over the common medium.

In the system of my invention, all the chan- .nels of communication, in pairs, are available to every ofiice. In order to reach a distant office, Ia r1 idle pair of channels is selected and a fre- 0 quency combination code is transmitted therelover to actuate signaling'devices at the desired distant-ofiice only, the call being completed thereat manually byanoperator. 7 Referring to the Figs. land 2 placed in jux-j taposition witheachother, the operations involving the setting up of a connection will now be described, 'itzbeing evident from the following description that the two trunks are perfectly isymmet'ricaland can be used either for originat- '50 .ingaaconnection or for completing a connection.

Itwill beassumed, therefore, that the connection is to be established between subscriber I00 the trunk in ofiice Ais normally connected to receive on, say, frequency h, and transmit on, say, frequency ii. In order to cooperate with similar trunks at other ofiices, these frequencies are reversed when the trunk is used for an outgoing call.

The operator at ofiice A, having associated plug PL1 of cord C with the subscribers line I00, and

the power supply PS to the oscillator 081, which produces frequency f1, and to oscillator 082 which produces frequency f2 and also to the modulator MD. It further completes a circuit for relay RLs extending from groundon the bottom outer contacts of relay RLs, bottom outer contacts of relay -RL1, winding of relay RLs to grounded battery.

Relay RLs operatesiand locks itself to operating ground through its No. 1 contacts independently of the contacts of relay RLI, thus causingit to remain energized as long as plug PLz remains in the jack JA. Relay RLe, through its Nos. 4 and contacts, connects oscillator CS1 to the modulator MD which, in turn,is connected to the band filter BF1 over the bottom front contacts of said relay, said filter passing frequency 3, while over its Nos. 2 and 3 front contacts, band filter BFz, which passes frequency f2, is connected to the demodulator DM. This changeover enables the trunk to transmit to and receive from any other trunk operating on these two frequencies. Inasmuchas the oscillator CS1 is connected to the Nos. 4 and 5 front contacts of relay RLG and the oscillator CS2 is connected to the back contacts of the same contact combination, carrier frequency I1 is transmitting to the medium.

This carrier frequency f1 indicates that the channel is in use and, is eife'ctive at other trunks in the system operating on the same set of frequencies, f1 and f2, as for instance, the trunk at ofiice B, to pass through filter BF1, lower back contacts of relay RLs, the demodulator DM1, and completing the circuit through the tertiary winding of thehybrid transformer KB. The demodulated current at each of the trunks causes the actuation of relay RL'1, which is bridged across the tertiary of said transformer, thereby completing a circuit for busy lamp BL extending from ground on the bottom inner contacts of said relay, bottom outer back contacts ofrelay RL'4, lamp BL, to grounded battery. The lighting of the lamp indicates that the trunk is in use while the opening of the bottom outer contacts of relay RL1 makes it impossible to operate relay RL's over a circuit similar to that described for relay RLs of the trunk in office A should an operator disregard the signal and insert a cord plug into jack JA in preparation'for using the trunk to establish an outgoing connection. The trunks at every oflice in the system, and operating on the frequencies f1 and f2 except at the originating office A when the corresponding trunk has been taken into use, are, therefore, in the condition just described, with the busy lamp BL lighted at every position to which the trunk is multipled. This condition is maintained at all offices except the called office as described hereinafter, until the channel is released at'the calling station.

Returning, now, to the: operations inthe'trunk Through at the calling office A, relay RLs, on operating, disconnects the voice frequency filters TCc and Ted from the demodulator DM while the bottom middle contacts close an obvious circuit to the busy lamps BL, which light at every position to which the trunk is multipled to indicate that the trunk is busy.

The carrier frequency f1 having been transmitted over the common medium and hence to the called oflice B as well as to every other office connected to the medium having a trunk operating on the fixed frequencies f1, f2, the operator at the calling ofiice A depresses one of the keys in the code sending apparatus CSA, such as B corresponding to the called ofiice. This apparatus may comprise a key-set having a key for each office associated with the common medium that may be reached by oflice A. To each of the keys is wired the combination of two sources of low or voice frequency currents which characterize the desired office. In Fig. 1, a code sending key device is shown having five separate keys B, C, F, each designating one of five distant ofiices. Each of these ofiices has a low frequency code designation made up of the combination of any two of the frequencies fa, fb, fc and fa which are derived from the-oscillators 08a, 05b,

and fa, can be reached from any of the five oflices B, C, F so that its characteristic frequency combination is not available through any of the. keys at ofiice A. The filters T00 and TCd in the trunk of oificeA, which respond to these frequencies when present on the common medium, however, have been subscripted, respectively, with the letters 0 and w to indicate the fact that these filters are responsive to the code frequencies which characterize the oflice A and transmitted from any of the other five ofiices. In the same way, the filters in ofiice B shown in Fig. 2 have been subscripted respectively, with a, and b to indicate that when frequencies a and b are on the medium only the trunks in oifice B equipped with filters T02. and TCb will respond.

While the scheme of ofiice identification is here being illustrated with a total of six oifices associated with the common medium, with each office being identified by an individual combination of two separate frequencies, it is understood that the invention is not limited to the number ,of offices which can be identified by combinations of two out of four availablefrequencies. It is obvious that the number of available frequencies, and the combinations possible with such frequencies, may be expanded to suit the number of ofiices which may be connected to the common medium, and the key-sets at each office may be expanded tohave as.many individual keys as there are offices to be reached, each of the keys carrying an appropriate number of contacts for connection to the appropriate number of frequency outlets. ,My invention, therefore, is intendedto embrace a communication system the oflices of which may be identified by a combination frequency code which identifies each of the separate oflices regardless of the number of ifice,which effects the release of relay RLs and officesinvolved.

Assume, now, that the two frequencies for the office B are the voice frequency is and. the voice frequency fb. When keyB isdepressed and key KY in the cord is depressedto the right, these two frequencies are modulated-upon carrier frequency ii, are transmittednover the common medium and are received at every office through the trunks thereat whichareadapted. to respond to carrier frequency f1. At office B, after demodulation, the two voice frequencies will pass the filters or tuned circuits Tea and TCb, respectively, and, after being rectified by therectifiers RF'1 and RFz will operate relays RLz and RL'a. The operation of these two relays completes an obvious circuit for relay RL4 which looks itself up over its bottom inner contacts and the bottom inner contacts of relay RL's. Relays RL'z and RLa will drop back after key B is released, that is, after the voice frequency code fa, it has been transmitted. Relay RL4, onoperating, opens the circuit of the busy lamp BL at its bottom outer back contact and, over its bottom outer front contact, completes the circuit of the calling lamp CL to the bottom inner contacts of relay RL'1. The circuit now remains in this condition until an'operator at the oifice B answers the calling signal by inserting a plug, such as PL'z into the jack JA, which completes the circuit for relay RL5 from ground through the cord supervisory lamp S, the sleeve of the plug PL'2, sleeve of the jack JA, winding of relay RLs, to grounded battery. Relay RL5 operates and connects the power supply circuit PS to the oscillators OS'i and OSz and modulator MD over its top middle contacts, opens the path of the tuned circuits TCa and TCb over its top outer contacts, opens the locking circuit of relay RL'4 over its bottom inner contacts, causing this relay to release and extinguish calling lamp CL, closes the circuit of busy lamp BL thereby lighting this lamp again to indicate the busy condition of the trunk, and further completes the path of the order tone circuit OT extending from ground on the top contacts of relay RL1, top contacts of relay RL4, top inner contacts of relay RL'5 to the order tone coil (not shown). Although the locking circuit of relay RL'4 was opened at the instant relay RL5 operated, said relay RL4 does not release immediately, it having slow-release characteristics, but persists in its operated condition for some time after its locking circuit is opened. This has the effect of sending a brief signal to the calling operator at oflice A by way of the order tone circuit, which will be heard in the operators telephone circuit at office A as a signal to pass the wanted subscribers number. Upon receipt of the number the operator at oflice B completes. the call by plugging PL1 of the cord C into the jack of the wanted subscriber 200.

Communication between subscribers is now carried over the carrier frequencies f1 and f2, calling subscriber H transmitting over frequency ]1 by virtue of the fact that relay RLs is operated and putting frequency h on the medium from oscillator CS1, and called subscriber 200 transmitting over frequency ,fz by virtue of the fact that relay RL'e is released and putting the frequency f2 on the medium from oscillator OS2. Supervision of the call is obtained by means of apparatus associated with the cord circuits in a Well-known manner.

After the call has been completed, the plug PLz is removed from the jack JA at the originating the removal of carrier. frequency h from the medium. I RelayRL'i at the called ofiice thereby releases and causes lampCL tobe extinguished as a signal to the operatorthat the call is. ter- ,oflice A (this relay having been operated when frequency is was placed on the medium) and extinguishing lamp BL to indicate that the trunk has been released at office B thus restoring the apparatus of the trunk at office A to normal in readiness for another call.

What is claimed is:

1. In a high frequency communicating system having a plurality of stations, means at each station for transmitting the same communicating carrier wave, means at each station for receiving the carrier wave transmitted by each of the other stations, sources of voice frequency currents at each station, key-controlled means for modulating said carrier wave with a combination of voice frequency currents to indicate a desired station, and means at said desired station responsive to said modulated wave for identifying said desired station.

2. In a carrier telephone system, a plurality of stations, a group of terminals at each of said stations, corresponding terminals at each station all being adapted for operative association with the same communicating channel, a call signal for each terminal at each of said stations, key-controlled means effective upon the use of one of the terminals at a station to extend .a call to another station for modulating the corresponding communicating channel with one or more lower frequency waves designating said other station, and meansin the terminal at said other station which is adapted for operative association with the same communicating channel as the terminal at the calling station used to extend the call responsive to said modulated carrier wave to operate the signal of said terminal at said other station.

3. In a high frequency communicating system having a plurality of stations, means at each station for transmitting and receiving the same pair of carrier frequencies, key-controlled means at each station for modulating one of said carrier frequencies by a combination of one or more lower frequencies to designate a wanted station, and means at each station responsive to the modulated carrier said means comprising a tuned device for each lower frequency assigned to the station, a rectifier operatively associated with each of said tuned devices, a relay for each rectifier and responsive to the rectified current, and a signal device responsiveto all of said relays when operated to signal the selection of the station.

4. In a high frequency communicating system having a plurality of stations, means at each station for transmitting the same pair of carrier waves, means at each station for receiving the carrier waves transmitted by each of the other stations, voice frequency means at each station for modulating one of said carrier waves to indicate a desired station, said means comprising a key for each station and sources of amplified voice frequency currents connected to the contacts of each of said keys in the combinations required by each of the stations identified by each of the keys, and means at each station responsive to the carrier wave modulated by the combination of voice frequencies which designate its own station for operating a signal to indicate the presence of a call for said station.

' 5. In a high frequency communicating system having a plurality of stations, a plurality of terminals at each station and a carrier channel available for one of said terminals at each of said stations, the combination at each station of key-controlled devices capable of operative association with each of the terminals thereat 

